A low noise amplifier (LNA) is an electronic amplifier used to amplify weak signals that are, for example, captured by an antenna of a radio communication device, such as a mobile phone. A low noise amplifier is an essential component in a Radio Frequency Integrated Circuit (RFIC), wherein it is typically the first amplifying block. Therefore, in addition to the targeted gain, noise and linearity performances, it should provide sufficient input matching to provide termination impedance for preceding blocks like RF/duplex filters. To be able to support an adequate number of radio frequency bands or just shortly bands, within a single RF-platform, several low noise amplifiers are needed. Depending on the operating mode, each low noise amplifier requires one or two receiver pins. The expression operating mode means in this application the operating configuration regarding receiving pins, wherein one or two pins can be used or both pins can be turned off. In addition to requirements mentioned above there may be additional requirements, such as a need for a Diversity receiver, which mandates even more receiver pins.
The target is to keep the number of pins as small as possible due to several obvious reasons. The RFIC should be as small as possible, and the design can then be pin or connection pad count limited, i.e. although the separation of connection pads is kept small, there is a limit for the maximum number of pins that the RFIC can conveniently support without compromising installation reliability. In the future, the pad count limitation might become a bottleneck, for example due to carrier aggregation that can increase the number of RF inputs, interface and control pins required. Also, when the number of RF pins is large, the routing on printed wiring board (PWB) and redistribution (RDL) layers becomes troublesome. Therefore, there might be certain inputs where the performance is traded off with cumbersome and longish signal routing paths. The situation described above is illustrated in FIG. 1.
To mitigate the input pin count problem, the recent trend has been towards single-ended receiver pins and low noise amplifiers. The single-ended (SE) low noise amplifier topology clearly requires less input pins than the differential one. The drawback is the sensitivity to interference; the balanced/differential structure senses the same common-mode interference in both branches, which is then reduced in the low noise amplifier output, and the interference can be tolerated. In the single-ended case, the interference cannot be separated from the wanted signal, and desensitization can happen. However, single-ended low noise amplifiers typically have a better dynamic range in same operating conditions. Therefore, there is a trade-off between the performance and the number of input pins required when choosing the configuration for the low noise amplifier. Therefore, in the beginning of the RFIC program, a selection between single-ended/differential low noise amplifiers has to be made. It is difficult to change the topology after the first engineering samples have been manufactured.
In FIGS. 2a-2f, examples of single-ended low noise amplifier topologies are illustrated. In addition, any single-ended low noise amplifier topology can be duplicated and thus modified into a differential one. In cellular products, low noise amplifiers are seldom truly single-ended but comprise either a passive or active balun. Conversion from single-ended to differential can be done after the low noise amplifier as well, but at some stage before the down-conversion mixer. The receiver front-end shall comprise a differential signal path, since typically a double-balanced mixer structure (both differential RF and LO ports) offers adequate performance for cellular products. In the figures, the following topologies are shown as examples. In FIG. 2a, a balun is included in the low noise amplifier input. In FIG. 2b, a balun is at the LNA output (load). In FIG. 2c, a signal is taken from a source, and drain nodes have a 180 degrees phase shift. In FIG. 2d, the voltage signal after the first common-source (CS) stage (MINA) is sensed with MINB. The balanced signal is (coarsely) achieved since the common-source stage turns the phase by 180 degrees. FIG. 2e discloses a topology with a balanced signal path created with a combination of common-source and common-gate stages. In FIG. 2f, the gate of MIN,CG is alternatively connected to the output of the CS stage (drain of MCS).
Thus, the problem of the prior art is how to maintain the same performance with a reduced number of pins required. The problems in a large number of pins include, for example, increase in space requirement, manufacturing costs and inflexibility in design.